trends in ozone in la 1960 to 2010 ... - university of utah

Trends in ozone, its precursors, and related secondaryoxidation products in Los Angeles, California: A synthesisof measurements from 1960 to 2010

Ilana B. Pollack,1,2 Thomas B. Ryerson,2 Michael Trainer,2 J. A. Neuman,1,2

James M. Roberts,2 and David D. Parrish2

Received 14 March 2013; revised 3 May 2013; accepted 6 May 2013; published 13 June 2013.

[1] Decreases in ozone (O3) observed in California’s South Coast Air Basin (SoCAB) overthe past five decades have resulted from decreases in local emissions of its precursors,nitrogen oxides (NOx=NO+NO2) and volatile organic compounds (VOCs). Ozoneprecursors have been characterized in the SoCAB with measurements dating back to 1960.Here we compile an extensive historical data set using measurements in the SoCABbetween 1960 and 2010. Faster rates of decrease have occurred in abundances of VOCs(�7.3� 0.7% yr�1) than in NOx (�2.6� 0.3% yr�1), which have resulted in a decrease inVOC/NOx ratio (�4.8� 0.9% yr�1) over time. Trends in the NOx oxidation productsperoxyacetyl nitrate (PAN) and nitric acid (HNO3), measured in the SoCAB since 1973,show changes in ozone production chemistry resulting from changes in precursor emissions.Decreases in abundances of PAN (�9.3� 1.1% yr�1) and HNO3 (�3.0� 0.8% yr�1) reflecttrends in VOC and NOx precursors. Enhancement ratios of O3 to (PAN+HNO3) show nodetectable trend in ozone production efficiency, while a positive trend in the oxidizedfraction of total reactive nitrogen (+2.2� 0.5% yr�1) suggests that atmospheric oxidationrates of NOx have increased over time as a result of the emissions changes. Changes in NOx

oxidation pathways have increasingly favored production of HNO3, a radical terminationproduct associated with quenching the ozone formation cycle.

Citation: Pollack, I. B., T. B. Ryerson, M. Trainer, J. A. Neuman, J. M. Roberts, and D. D. Parrish (2013), Trends inozone, its precursors, and related secondary oxidation products in Los Angeles, California: A synthesis of measurementsfrom 1960 to 2010, J. Geophys. Res. Atmos., 118, 5893–5911, doi:10.1002/jgrd.50472.

1. Introduction

[2] Ozone concentrations have decreased significantlysince the 1960s in the California South Coast Air Basin(SoCAB), a region encompassing the Los Angeles urbanarea. Maximum 8 h average ozone mixing ratios measuredbasin wide and at individual surface monitoring network sta-tions in the SoCAB decreased by about a factor of 3(Figure 1; http://www.arb.ca.gov/adam/index.html) between1973 and 2010. The observed decrease in ozone is attributedto decreases in local emissions of volatile organic com-pounds (VOCs) [Warneke et al., 2012] and nitrogen oxides(NOx =NO+NO2) [McDonald et al., 2012], the precursorsto ozone formation. Reactions of hydrocarbons and carbonmonoxide (CO) with hydroxyl radicals (OH), as shown in

(R1) and (R2), initiate ozone formation chemistry[Finlayson-Pitts and Pitts, 2000; Jacob, 1999] through for-mation of peroxy radicals. Oxidation of NO by HO2 or RO2

via (R3)–(R5) then generates nitrogen dioxide (NO2) andproduces ozone following photolysis (R6) and reaction withmolecular oxygen (R7).

HOþ COþ O2 ! HO2 þ CO2 (R1)

HOþ RHþ O2 ! RO2 þ H2O (R2)

HO2 þ NO ! NO2 þ HO (R3)

RO2 þ NO ! NO2 þ RO (R4)

RO2 þ NOþ O2 ! R0CHOþ NO2 þ HO2 (R5)

NO2 þ hn ! Oþ NO (R6)

Oþ O2 þM ! O3 þM (R7)

[3] Ozone precursors have been extensively studied overthe years in the SoCAB. Some of the earliest measurementsof NOx, CO, and speciated VOCs in downtown Los

1Cooperative Institute for Research in Environmental Sciences,University of Colorado Boulder, Boulder, Colorado, USA.

2Chemical Sciences Division, NOAA Earth System ResearchLaboratory, Boulder, Colorado, USA.

Corresponding author: I. B. Pollack, Chemical Sciences Division, EarthSystem Research Laboratory, National Oceanic and AtmosphericAdministration, 325 Broadway, MS R/CSD7, Boulder, CO 80305, USA.([email protected])

©2013. American Geophysical Union. All Rights Reserved.2169-897X/13/10.1002/jgrd.50472

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Angeles date back to 1960 [Neligan, 1962], after emissionsfrom automobiles and local industry were identified as majorcontributors to photochemical smog [Haagen-Smit, 1952;Haagen-Smit and Fox, 1954]. These measurements initiatedthe development of an extensive surface monitoring networkin the state of California and motivated future intensive fieldmeasurement campaigns. Since 1960, several short-termground-based field studies have been conducted at selectedlocations within the SoCAB. Basin-wide measurements frominstrumented research aircraft began in the 1970s [Husaret al., 1977], and near-tailpipe measurements frommobile roadside monitors began in the early 1990s [Beatonet al., 1995; Bishop and Stedman, 2008; Gertler et al.,1999; Lawson et al., 1990]. Long-term trends in ozone andemissions of its precursors in the SoCAB have been exten-sively studied using the data collected in these experiments[Ban-Weiss et al., 2008; Bishop and Stedman, 2008;Dallmann and Harley, 2010; Fortin et al., 2005; Fujitaet al., 2003; Fujita et al., 2013; Grosjean, 2003; Harleyet al., 2005; McDonald et al., 2012; Parrish et al., 2002;Parrish et al., 2011; Warneke et al., 2012].[4] Other secondary pollutants such as nitric acid

(HNO3), alkyl nitrates (RONO2), peroxides (H2O2 andROOH), and peroxyacetyl nitrate (PAN; CH3C(O)O2NO2), formed in reactions accompanying those that pro-duce ozone, have also been measured in the SoCAB.Formation of HOx-NOx oxidation products, such as nitratesvia (R8) and (R9) and peroxides via (R10) and (R11), effec-tively removes these radicals from ozone-producing reac-tion cycles. In contrast, formation of VOC-NOx oxidationproducts, such as PAN via (R12), produces temporary reser-voir species for HOx and NOx due to relatively short thermaldecomposition lifetimes for the peroxyacyl nitrates undersurface conditions characteristic of summertime in LosAngeles [Roberts et al., 2007; Roberts et al., 1995;Stephens, 1969; Grosjean et al., 2001; Grosjean, 2003;Tuazon et al., 1991]. These reactions propagate the radicalchain and lead to continued ozone production.

HOþ NO2 ! HNO3 (R8)

RO2 þ NO ! RONO2 (R9)

HO2 þ HO2 ! H2O2 þ O2 (R10)

RO2 þ HO2 ! ROOHþ O2 (R11)

CH3C Oð ÞO2 þ NO2 ! CH3C Oð ÞO2NO2 (R12)

[5] The interdependence of these odd-hydrogen sinks withthe chain reactions that produce ozone, reactions (R1)–(R7),makes the oxidation products of reactions (R8)–(R12) auseful tool for attributing a cause and effect relationshipbetween decreasing ozone and decreasing precursor abun-dances [Kelly, 1992; Roberts et al., 2007; Roberts et al.,1995; Sillman et al., 1990; Sillman, 1991; Trainer et al.,1993]. Even though secondary oxidation products have beenmeasured and studied in SoCAB field experiments since the1960s and 1970s, relatively few peer-reviewed publicationsanalyze these data over multiple years. These few includereviews of hydrogen peroxide (H2O2) measurements [Leeet al., 2000; Sakugawa et al., 1990], with several measure-ments in the California SoCAB between 1970 and 1988,and reports of ambient PAN measurements at various sitesin the SoCAB between 1960 and 1997 by Grosjean [2003]and between 1975 and 1983 by Temple and Taylor [1983].Historical analyses of secondary oxidation products otherthan ozone in the SoCAB have been largely neglected, de-spite their key role as indicators for understanding the re-sponse of ozone production rates and yields followingchanges in precursor emissions.[6] In this work, we confirm and extend reported long-term

trends in abundances and emission ratios of ozone precursorsover the five decades from 1960 to 2010 in the SoCABusing data from surface monitoring network stations,mobile roadside monitors, ground-based field campaigns,and instrumented research aircraft. Abundances and emissionratios of ozone precursors determined from these measure-ments are also compared to those derived from emissioninventories. We further report long-term trends in secondaryoxidation product concentrations and extend the measure-ments to 2010. Measured abundances and enhancementratios for secondary oxidation products are compared withvalues predicted by a chemical box model [Fujita et al.,2013]. These long-term trends are described as exponentialdecreases [e.g., Parrish et al., 2002; Warneke et al., 2012]and quantified by calculating average rates of change peryear. We use this approach to define trends in measuredprecursors and secondary pollutants. Correlations of ozoneand related oxidation products with precursor abundancesand VOC/NOx ratio are identified using these historical data.Decadal changes in ozone production efficiency and rate ofphotochemical processing are interpreted from long-termtrends in ozone and NOx oxidation products.

2. Methods

2.1. Data Sets

[7] Data for this analysis are compiled from measure-ments performed in the SoCAB between 1960 and 2010.

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2

3

4

5

Max

8-h

r av

erag

e O

3 (p

pbv)

20102000199019801970Year

Figure 1. Maximum 8 h average ozone mixing ratios fromselected Air Quality Monitoring District (AQMD) monitor-ing network stations in the SoCAB. Each data point repre-sents the maximum 8 h average for a 1 year period. LLS fitof the maximum 8 h average for all sites in the SoCAB (solidgray line) indicates a decrease in ozone by a factor of 2.9 be-tween 1973 and 2010, corresponding to a rate of decrease of2.8� 0.2%yr�1.

POLLACK ET AL.: OZONE TRENDS IN LA FROM 1960 TO 2010

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Measurements are acquired from (1) the South Coast AirQuality Monitoring District (SCAQMD) and PhotochemicalAssessment Monitoring Stations (PAMS) surface network,(2) mobile roadside monitors, (3) ground-based field studies,and (4) chemically instrumented research aircraft. Figure 2illustrates the locations of the network monitoring sites andground-based field study locations as well as example flighttracks from the airborne experiments used in this analysis.2.1.1. Surface Network Stations[8] Trace gas data since the 1970s were acquired from the

web-based Air Quality Data Statistics database maintainedby the California Air Resources Board (CARB) (http://www.arb.ca.gov/adam/index.html) for select monitoring stations inthe SoCAB network. These data include annual 8 h maximumozone concentrations; hourly measurements of NO, NO2, and

CO; and 3 h canister samples of volatile organic compounds(VOCs). We focus on data collected from the Azusa andUpland sites due to their long-term data coverage and proxim-ity to ground-based field study locations; data coverage andmeasurement techniques at these stations are summarized inTable 1. We use the available NO2 data from these groundsites despite a known sensitivity of the NO2 measurements toorganic nitrates and HNO3, which depends upon inlet config-uration and thermal operation range of a molybdenum con-verter [Fehsenfeld et al., 1990; Fitz, 2002; Murphy et al.,2007; Winer et al., 1974]. NOx mixing ratios are calculatedas the sum of the reported NO and NO2 measurements.Since precursors in the SoCAB primarily arise from motorvehicle emissions, we focus our analysis on VOC speciescharacteristic of automobile exhaust, including benzene,

Figure 2. Measurements for this analysis are compiled from surface network sites (white circles), road-side monitors (cyan squares), ground-based field sites (yellow triangles), and airborne studies (sample flighttracks in pink). Airborne measurements collected in the mixed boundary layer (<1 km) over the SoCAB(dashed box) are interpreted as basin-wide averages.

Table 1. Summary of Trace Gas Data From the Azusa and Upland Network Stations in the SoCAB

Measurement Site Data Coverage Techniquea

VOCb Azusa 1974–1975, 1995–2010 Canister collection followed by gas chromatographyUpland 1975, 1994–2008 Canister collection followed by gas chromatography

O3 Azusa 1978–2010 Ultraviolet absorptionUpland 1975–2010 Ultraviolet absorption

CO Azusa 1975–2010 Nondispersive infrared absorptionUpland 1975–2010 Nondispersive infrared absorption

NO Azusa 1975–1979 ColorimetryAzusa 1980–2010 Ozone-induced chemiluminescenceUpland 1975–2010 Ozone- induced chemiluminescence

NO2c Azusa 1975–1979 Colorimetry using Lyshkow-modified Saltzman reagent

Azusa 1980–2010 Heated molybdenum converter followed by chemiluminescenceUpland 1975–2010 Heated molybdenum converter followed by chemiluminescence

aAdditional information about measurement techniques and instrumentation are available on the CARB website (http://www.arb.ca.gov/airwebmanual/in-dex.php).

bVOC measurements (non-methane organic carbon (NMOC) in the original data files) represent 3 h samples collected on a varying schedule during a3month period from July through September. Years represent data coverage for benzene, toluene, ethylbenzene, and o-xylene; measurements of isopreneare only available at Azusa and Upland between 1994 and 2008.

cNO2 measurements since 1980 at Azusa and 1975 at Upland include a known sensitivity to organic nitrates and HNO3, which depends upon inlet config-uration and thermal operation range of the molybdenum converter [Fehsenfeld et al., 1990; Fitz, 2002;Murphy et al., 2007;Winer et al., 1974]; NOx is cal-culated as the sum of the NO and NO2 measurements.


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toluene, ethylbenzene, and o-xylene. Measurements ofisoprene are used to assess potential changes in biogenic emis-sions over time. Although network measurements of VOCs(also called non-methane organic compounds (NMOCs) inthe data sets) date as far back as 1990 [Warneke et al., 2012],we only utilize quality-reviewed data since 1994 for this anal-ysis. Network measurements of VOCs below the instrumentallimit of detection are reported by CARB as one-half the limit ofdetection. Measurements of VOCs are converted from units ofparts per billion of carbon (ppbC) to parts per billion by volume(ppbv) of compound prior to further analysis.2.1.2. Roadside Monitors[9] Ozone precursor emissions from light-duty and heavy-

duty vehicles measured by mobile roadside monitors inCalifornia are available from peer-reviewed publications[Bishop and Stedman, 2008; Bishop et al., 2010; Bishopet al., 2012; Stedman et al., 2009] and research reports acces-sible from the Fuel Efficiency Automobile Test (FEAT)website (http://www.feat.biochem.du.edu). We use measure-ments of NO, CO, and total unspeciated hydrocarbons(total HC) including methane from light-duty vehiclesacquired at the Los Angeles, Van Nuys, and Riversidemonitoring sites since 1991 [Bishop and Stedman, 2008;Bishop et al., 2010]; measurement periods and locationsare summarized in Table 2. For this analysis, weassume that NOx is primarily emitted as NO [Soltic andWeilenmann, 2003] and that there is negligible chemicalprocessing between emission and detection given the sam-pling configuration of the instrument [Burgard et al.,2006]; roadside measurements of NO are thus referred toas NOx in the following analysis. Molar emission ratios ofCO, NOx, and (total HC) to CO2 are derived from thereported fuel-specific emissions of grams of pollutant perkilogram of fuel [Bishop and Stedman, 2008; Burgardet al., 2006; Singer et al., 1998].2.1.3. Ground-Based and Airborne Field Studies[10] The SoCAB has been the focus of many ground-based

and airborne air quality field campaigns. Compound-specificVOCs, NOx, and CO and secondary oxidation products suchas O3, PAN, and HNO3 have beenmeasured during these stud-ies. The species measured, measurement techniques, data cov-erage, sampling locations, and references for field studiesperformed between 1960 and 2010 are summarized in Table 3.

[11] Our historical analysis of precursor emissions beginswith ground-based measurements of VOCs and NOx indowntown Los Angeles in 1960 by Neligan [1962]. Thisstudy was followed with additional measurements by theCalifornia Air Pollution Control District in 1963 [Leonardet al., 1976], Lonneman et al. [1968] in 1966, Altshulleret al. [1971] in 1967, and Kopczynski et al. [1972] in 1968.Data from these studies were acquired from tables and textwithin the cited literature.[12] More extensive ground-based field studies supported

by CARB and SCAQMD began in 1987 with the SouthernCalifornia Air Quality Study (SCAQS) and continued withthe Los Angeles Atmospheric Free Radical Study(LAAFRS) in 1993 and the Southern California OzoneStudy (SCOS) in 1997. SCOS 1997 incorporated measure-ments from several instrumented aircraft, although we usemeasurements only from the Piper Aztec aircraft due to itsprimary sampling objective to characterize ozone andprecursors in the SoCAB [Croes and Fujita, 2003].Archived data from field projects supported by CARB wereacquired directly from CARB. Peer-reviewed publicationsoutlining objectives, measured species, measurementtechniques, sampling details, and results are available formost field studies; research reports are available for theremainder on the CARB website (http://www.arb.ca.gov/re-search/research.htm).[13] Airborne measurements are also compiled from

several field campaigns performed in the SoCAB in the pastdecade. A chemically instrumented NOAA P-3 aircraftsampled the daytime mixed layer throughout the LosAngeles Basin on 13 May 2002 during the IntercontinentalTransport and Chemical Transformation (ITCT) study[Parrish et al., 2004], and the NASA DC-8 aircraft sampledthe same general area on 18 June 2008 during the Californiaphase of the Arctic Research of the Composition of theTroposphere from Aircraft and Satellites study(ARCTAS-CARB) [Jacob et al., 2010]. Most recently,the California Research at the Nexus of Air Quality andClimate Change (CalNex) field project in May, June,and July 2010 reported chemical measurements in theLos Angeles mixed layer from the NOAA P-3 aircraftand from a ground site in Pasadena, California [Ryersonet al., 2013]. Archived data for the field studiesconducted by NOAA and NASA are publicly availableon each institution’s website (http://www.esrl.noaa.gov/csd/field.html; http://www-air.larc.nasa.gov/missions.htm).[14] Airborne and ground-based measurements of second-

ary oxidation products, such as PAN and HNO3, are avail-able from SCAQS 1987, SCOS 1997, ITCT 2002,ARCTAS-CARB 2008, and CalNex 2010. Additional mea-surements are reported in the literature for ground-based fieldstudies near Riverside in 1962 [Darley et al., 1963] and 1982[Russell et al., 1988a]; Los Angeles in 1968 [Kopczynskiet al., 1972]; Pasadena in 1973 [Hanst et al., 1982]; WestCovina in 1973 by Spicer [1977b]; Claremont in 1978 byTuazon et al. [1981] and 1985 by Grosjean [1986, 1988];Glendora in 1986 by Anlauf et al. [1991]; and TanbarkFlats in 1990 and 1991 by Grosjean et al. [1993].[15] Ground-based field measurements of hydrogen perox-

ide (H2O2) are available from the Carbonaceous SpeciesMethods Comparison Study (CSMCS) at Glendora in 1986[Kok et al., 1990; Mackay et al., 1990; Sakugawa and

Table 2. Summary of Roadside Measurements in the SoCAB

Year Dates Location Reference

1991 19 May to 27Jun

LosAngeles

Beaton et al. [1995]

1999 15 Oct to 19 Oct LosAngeles

Bishop and Stedman [2008]

28 Jun to 7 Jul Riverside2000 30 May to 6 Jun Riverside Bishop and Stedman [2008]2001 15 Oct to 19 Oct Los

AngelesBishop and Stedman [2008]

6 Jun to 13 Jun Riverside2003 27 Oct to 31 Oct Los

AngelesBishop and Stedman [2008]

2005 17 Oct to 21 Oct LosAngeles

Bishop and Stedman [2008]

2008 17 Mar to 21Mar

LosAngeles

Bishop et al. [2010]; Stedman et al.[2009]

2010 12 Aug to 16Aug

Van Nuys Bishop et al. [2012]


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Table 3. Summary of Measurements From Airborne and Ground-Based Field Studies in the SoCAB

Year Dates Location Measurement Technique References

1960 18 Aug to18 Nov

Los Angeles NOx, COi Nearby monitoring station Neligan [1962]

Benzene, toluenei Gas chromatographyPANj Polyethylene bag collection followed by long-

path Fourier transform infrared spectroscopyRenzetti and Bryan [1961]

1962 N/A Riverside PANj Electron capture-gas chromatography Darley et al. [1963]1963–1965

20 Apr to6 Nov

Los Angeles Benzene, toluenej Gas chromatography Leonard et al. [1976]

1966 1 Sep to30 Nov

Pasadena/ NOx, COj Nearby monitoring station Lonneman et al. [1968]

Los Angeles Toluene, ethylbenzene,o-xylenej

Tedlar bag collection followed by gaschromatography

1967 19 Sep to17 Nov

LosAngeles/Azusa

Toluene, ethylbenzene,o-xylenej


Altshuller et al. [1971]

1968 5 Sep to13 Nov

Los Angeles NOxj Chromium trioxide paper and Griess-Saltzman

reagentKopczynski et al. [1972];Lonneman et al. [1976]

Toluene, ethylbenzene,o-xylene, COj


PANj Gas chromatography-electron capture detection Lonneman et al. [1976]1970 7, 10 Aug Riverside H2O2

j Teflon bag collection followed by colorimetrywith Ti (IV)

Bufalini et al. [1972]

1971 23 Aug to14 Oct

Los Angeles Benzene, toluenej Gas chromatography Leonard et al. [1976]

1973 2 Jul to 30 Aug Los Angeles Benzene, toluenej Gas chromatography Leonard et al. [1976]1973 24 Jul to

26 JulPasadena PANi Long-path Fourier transform infrared

spectroscopyHanst et al. [1975]

1973 24 Aug to28 Sep

West Covina O3, NO, NOxi Chemiluminescence Spicer [1977a]; Spicer [1977b]

PANi Gas chromatography-electron capture detectionHNO3

i Acid-detecting mast colorimetry1977 19 Jul to

29 JulClaremont/Riverside

H2O2j Scrubbing coil impinger followed by luminol-

based chemiluminescenceKok et al. [1978]

1978 9 Oct to13 Oct

Claremont NO, NO2i Chemiluminescence Tuazon et al. [1981]

O3, PAN, HNO3i Long-path Fourier transform infrared

spectroscopy1979 9 Apr to

21 AprLos Angeles Benzene, toluene,

ethylbenzene, o-xylenej

Canister/Gas Chromatography-flame ionizationdetector

Singh et al. [1981]

PANj Gas chromatography-electron capture detection1980 26 Jun to

27 JunLos Angeles CO, NO, NO2, NOx,

PAN, HNO3, O3i

Long-path Fourier transform infraredspectroscopy

Hanst et al. [1982]

1982 31 Aug Pasadena/Riverside

PANj Gas chromatography-electron capture detection Russell et al. [1988a]

1985a 11 Sep to18 Sep

Claremont O3 UV absorption (from nearby monitoring station)NO, NO2

i Chemiluminescence Grosjean [1988]PANi Gas chromatography-electron capture detectionHNO3

i Filter pack followed by ion chromatography; Winer et al. [1986]Fourier transform infrared spectroscopy

1986b 12 Aug to21 Aug

Glendora HNO3j Fourier transform infrared spectroscopy; Anlauf et al. [1991]

Tunable diode laser absorption spectroscopy;Filter pack followed by ion chromatography

H2O2j Dual coil impinger followed by dual enzyme

fluorescence;Kok et al. [1990]

Impinger with diffusion scrubber followed bydual enzyme fluorescence;

Tanner and Shen [1990]

Cryogenic trap followed by dual enzymefluorescence;

Sakugawa and Kaplan [1990]

Tunable diode laser absorption spectroscopy Mackay et al. [1990]1987c 19 June to

3 SepClaremont O3, NOx, NOy

k Chemiluminescence Fujita et al. [1992]; Lawson [1990];Williams and Grosjean [1990]

PANk Gas chromatography-electron capture detection Williams and Grosjean [1990]HNO3

k Tunable diode laser absorption spectroscopyCO, benzene, toluene,o-xylene, isoprenek

Canister/Gas Chromatography-flame ionizationdetector

H2O2j Tunable diode laser absorption spectroscopy Mackay et al. [1988]

1990 3 Aug to 5 Sep Tanbark Flat PANj Gas chromatography-electron capture detection Grosjean et al. [1993]1991 5 Aug to 12 Sep Tanbark Flat PANj Gas chromatography-electron capture detection Grosjean et al. [1993]1993d 1 Sep to

26 SepClaremont O3

k UV absorption Mackay [1994]NO2, NOx, NOy

k ChemiluminescenceCO, benzenek Canister and DNPH cartridge followed by gas

chromatography-flame ionization detectorPANk Gas chromatography/chemiluminescence

HNO3, H2O2k Tunable diode laser absorption spectroscopy

(Continues)


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Kaplan, 1990; Tanner and Shen, 1990], SCAQS 1987[Mackay, 1988], and LAAFRS 1993 [Mackay, 1994]; airbornemeasurements of H2O2 are available from ARCTAS-CARB2008 [Jacob et al., 2010]. Hourly measurements of H2O2 wereobtained upon request from CARB for LAAFRS, and 15 sH2O2 data were acquired from the NASA website forARCTAS-CARB. All other data points represent reported av-erages or maxima from the literature. Historical measurementsof H2O2 are summarized in Table 3. Earlier reports of H2O2

dating back to 1970 at Riverside [Bufalini et al., 1972] and1977 at Claremont [Kok et al., 1978] were later invalidated ow-ing to an interference of the H2O2 collecting solution withozone and possible aqueous reactions of H2O2 with other gases

[Kok et al., 1988; Lee et al., 2000]. Eliminating the data pointsfrom 1970 and 1977 leaves too fewmeasurements to identify asignificant trend; thus, further interpretation of H2O2 data inthe SoCAB is not attempted here.[16] A historical account of alkyl nitrates is also not reported

here owing to a lack of field measurements. For analysesrequiring total reactive nitrogen (NOy) in section 4.3, weassume alkyl nitrates are a relatively small contribution tothe sum of reactive nitrogen species in the Los Angeles basin.We support this assumption by comparing daytime measure-ments of gas-phase NOy with the calculated sum of NOy,where ΣNOy=NOx+PAN+HNO3 [Parrish et al., 1993],from four field studies between 2002 and 2010. Quantitative

Table 3. (continued)

Year Dates Location Measurement Technique References

1997e 16 Jun to 15 Oct Azusa O3k UV absorption Blumenthal [1999]; Croes and Fujita

[2003];Fitz [1999]; Fujita et al. [1999];

Grosjean [2003];Grosjean et al. [2001]

NOyk Chemiluminescence

CO, CO2, benzene,toluene, ethylbenzene,

o-xylenek

Canister and DNPH cartridge followed by gaschromatography-flame ionization detection

PANk Gas chromatography-electron capture detectionHNO3

k Tunable diode laser absorption spectroscopy

1997e 16 Jun to 15 Oct Piper Aztecaircraft

O3k UV absorption Croes and Fujita [2003]; Fitz [1999];

Fujita et al. [1999];Grosjean [2003];Grosjean et al. [2001]

NOyk Chemiluminescence

CO, CO2, benzene,

toluene, o-xylenek

Canister and DNPH cartridge followed by gas

chromatography-flame ionization detection

2002f 13 May NOAA P-3aircraft

O3, NOx, NOyl Chemiluminescence Parrish et al. [2004]

COl Vacuum UV resonance fluorescenceCO2

l Nondispersive infrared absorptionPAN, HNO3

l Chemical ionization mass spectrometrybenzene, toluene, o-

xylenelWhole air sampler followed by gas

chromatography2008g 13 May NASA DC-

8 aircraftO3, NOx, NOy

m Chemiluminescence Jacob et al. [2010]COm Vacuum UV resonance fluorescenceCO2

m Nondispersive infrared absorptionPAN, HNO3

m Chemical ionization mass spectrometrybenzene, toluenem Proton transfer reaction mass spectrometry

2010h 1 May to 30 Jun NOAA P-3aircraft

O3, NOx, NOyl Chemiluminescence Ryerson et al. [2013]


l Wavelength-scanned cavity ring-downspectroscopy

PAN, HNO3l Chemical ionization mass spectrometry

benzene, toluene,ethylbenzene,o-xylenel

Whole air sampler followed by gaschromatography

2010h 15 May to 15Jun

Pasadena O3l UV absorption Ryerson et al. [2013]

NOx, NOyl Chemiluminescence


l Nondispersive infrared absorptionPAN, HNO3

l Chemical ionization mass spectrometrybenzene, toluene,ethylbenzene,o-xylenel

Gas chromatography-mass spectrometry

aNitrogen Species Methods Comparison Study (NSMCS) 1985.bCarbonaceous Species Methods Comparison Study (CSMCS) 1986.cSouthern California Air Quality Study (SCAQS) 1987.dLos Angeles Atmospheric Free Radical Study (LAAFRS) 1993.eSouthern California Ozone Study (SCOS) 1997.fIntercontinental Transport and Chemical Transformation (ITCT) 2002.gCARB phase of Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS-CARB) 2008.hCalifornia Research at the Nexus of Air Quality and Climate Change (CalNex) 2010.iData reported in the cited literature.jAverage values reported in the literature reference.kData requested from CARB.lData acquired from NOAA website (http://www.esrl.noaa.gov/csd/field.html).mData acquired from NASA website (http://www-air.larc.nasa.gov/missions.htm).


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agreement within 1s uncertainties, which typically range from�10% to �20% of the mixing ratio, between measured NOy

and ΣNOy suggests little contribution on average from otherspecies to NOy during the most recent decade of our long-term analysis. An earlier account of alkyl nitrate contributionsin the SoCAB reported byGrosjean [1983] showedmixing ra-tios of methyl nitrate up to ~5ppbv during photochemicalsmog episodes in Claremont in 1980; however, methyl nitratecontributed <5% to ΣNOy since NOx, PAN, and HNO3 werealso significantly enhanced during this time period.[17] Accuracy of the utilized data is considered in this

analysis. For data sets found in the literature, we assume thatmeasurements were subjected to quality analysis and peerreview prior to publication and that any concerns with dataquality, measurement technique, instrumentation, or sam-pling were reported with the published data or in subsequentpublications. For field study data provided by CARB, weassume that final reported data have been subjected to qualityanalysis with questionable data points eliminated or flaggedappropriately. Where available, information regardingmeasurement uncertainties, precision, and possible interfer-ences by other species is taken from data file headers andsupporting documents.

2.2. Data Analysis

[18] This analysis is focused on measurements during sum-mertime, when ozone production is at a maximum, andweekdays, when precursor emissions are most prominent.Surface network measurements are limited to data collectedbetween 1 May and 30 September; field study data are aver-aged over the collection periods summarized in Table 3,which take place between May and November and range

from a single day to several months. With the exception ofthe aircraft campaigns where flight times are typically duringmidday, measurements of VOCs, CO, and NOx are furtherlimited to samples collected on weekday mornings between0500 and 0900 PDT when precursor emissions are at a max-imum and prior to development of a well-mixed planetaryboundary layer and extensive photochemical processing.Conversely, measurements of oxidation products, includingO3, PAN, and HNO3, are taken from weekday afternoonsbetween 1200 and 1800 PDT when the planetary boundarylayer is well developed and accumulated concentrations ofsecondary pollutants are at a maximum.[19] To simplify the analysis, we group the available

ground-based measurements from various locationsthroughout the SoCAB into two general geographic areas.The first includes sampling sites on the western side of theSoCAB located near major centers of emissions, such asLos Angeles, Pasadena, Azusa, and Glendora, and the sec-ond includes locations generally downwind, such asUpland, Claremont, and Riverside, on the eastern side ofthe basin. We distinguish data points acquired from thetwo defined areas for comparison. Airborne measurementsfrom the mixed boundary layer over the SoCAB between0.2 and 1 km above ground level, 33.6� < latitude< 34.3�,and �118.5� < longitude<�116.8� [Pollack et al., 2012],are retained for this analysis as a representative of basin-wide averages.[20] Previously reported values for average atmospheric

abundances and emissions ratios of ozone precursors andsecondary pollutants are taken from the literature whereavailable. Otherwise, we calculate average abundances andemissions ratios from the data sets described above.

Table 4. Rate of Change (Δ) and Corresponding 1s Standard Deviation in Units of Percent Change Per Year for Abundances andEmissions Ratios of Ozone and Its Precursorsa

Measurement

Field Observationsb Roadside Monitors AQMD Network—Azusa AQMD Network—Upland All Observationsc

Δ (% yr�1) r2 N Δ (%yr�1) r2 N Δ (% yr�1) r2 N Δ (% yr�1) r2 N Δ (% yr�1) r2 N

O3 (8 h max) �3.4� 0.2 0.88 33 �3.0� 0.1 0.92 38 �2.8� 0.1d 0.91 38NOx �5.2� 1.0 0.82 8 �2.0� 0.3 0.68 21 �1.8� 0.3 0.51 36 �2.6� 0.3 0.56 65CO �7.9� 0.8 0.95 7 �4.8� 0.4 0.89 21 �5.2� 0.4 0.87 26 �5.7� 0.3 0.86 54Benzene �8.6� 0.8 0.94 9 �7.4� 0.5 0.94 17 �7.6� 0.8 0.88 15 �8.0� 0.3 0.94 41Toluene �8.0� 0.8 0.91 11 �6.6� 0.4 0.95 17 �5.7� 0.6 0.86 16 �6.7� 0.3 0.92 44Ethylbenzene �8.6� 0.9 0.95 6 �5.8� 0.6 0.85 17 �4.2� 1.0 0.55 16 �6.9� 0.4 0.87 39o-Xylene �8.7� 1.0 0.94 7 �8.5� 0.6 0.93 17 �6.7� 0.7 0.87 16 �7.5� 0.3 0.93 40Isoprene 0.7� 2.1 0.01 14 0.3� 0.9 0.01 15CO/CO2 �11.6� 1.7 0.92 6 �9.9� 1.0 0.92 10 �11.5� 1.4e 0.82 16NOx/CO2

f �5.5� 2.6 0.52 6 �7.0� 1.0 0.86 9 �6.6� 1.7e 0.52 15(Total HC)/CO2 �11.6� 1.0 0.83 10Benzene/CO2 �7.7� 1.3 0.88 6NOx/CO

f 5.2� 1.4 0.59 12 5.9� 1.3 0.77 9 3.6� 0.7 0.68 17 5.5� 0.4 0.89 26 4.9� 0.4 0.71 67(Total HC)/CO �1.8� 2.3 0.07 10Benzene/CO �1.1� 0.6 0.30 10 0.7� 0. 6 0.08 17 �1.6� 0.4 0.70 8 �1.2� 0.4 0.20 35(Total HC)/NOx �2.0� 2.7 0.08 9Toluene/NOx �4.4� 0.7 0.86 9 �3.8� 0.7 0.68 17 �5.1� 0.9 0.69 16 �4.6� 0.4 0.78 42Ethylbenzene/NOx �4.5� 0.4 0.97 5 �3.0� 0.9 0.44 17 �5.9� 2.0 0.40 15 �4.2� 0.6 0.60 37o-Xylene/NOx �5.6� 0.7 0.92 7 �5.5� 0.7 0.80 17 �5.4� 1.8 0.40 15 �5.7� 0.5 0.77 39

aA negative rate of change signifies a decreasing trend over time; N represents the number of data points, corresponding to years of data, used forlinear regression.

bOverall trend since 1960 for ground-based observations of precursor abundances, overall trend since 1960 for airborne and ground-based observations ofprecursor emissions ratios.

cOverall trend since 1960 representing combined data from the available field, roadside, and network observations.dOverall trend since 1973 determined from maximum 8 h average ozone for all sites in the SoCAB (Figure 1).eOverall trend since 1991 representing a combined LLS fit of the available field and roadside observations.fNOy used for emissions ratios from airborne and ground-based field studies where available.


5899

Average abundances are calculated as the arithmetic mean ofall retained measurements for a given substance during thespecified collection periods. The corresponding uncertaintiesrepresent a confidence limit calculated by dividing the 1sstandard deviation of the mean by the square root of the num-ber of days of observations. Emissions ratios of directly emit-ted species, and enhancement ratios involving secondaryoxidation products, are determined by linear least squares(LLS) [Press et al., 1988] orthogonal distance regression(ODR) [Boggs et al., 1987] to the relevant observations; theslope of the fitted line is interpreted as the emissions or en-hancement ratio [Pollack et al., 2012]. Where informationis available, the LLS ODR fits are weighted by the impreci-sion of the measurements, and a total uncertainty for each ra-tio is calculated by quadrature additions of the uncertainty inthe fitted slope and the uncertainties of the respective mea-surements [Taylor, 1997]. Error bars associated with theseratios reflect measurement uncertainty as well as day-to-dayvariability. For data sets where instrument accuracy and pre-cision information are not reported, uncertainty in the emis-sions ratio is reported as the 1s standard deviation of theslope.[21] LLS regression analysis of the logarithmic-

transformed data is used to quantify the exponentialchange describing the trends in abundances and emissionsratios of ozone precursors (Table 4) as well as abun-dances and enhancement ratios of secondary oxidationproducts (Table 6). Extracting information about thetrends from a functional fit to multiyear data minimizesthe influence of interannual variability from various mea-surement sites, platforms, meteorological conditions, andpossible pollution episodes. Here long-term changes arequantified and reported in two ways: (1) a constant rateof change, R, reported in units of percent change per year,and (2) a factor of change, f, spanning the range of yearsof available measurements. We assume an exponentialfunction (1) for changes in abundances and emissionsratios over time.

yt ¼ y0 � e�at (1)

[22] Rates and factors of change for a given species, y, arethen derived from the slope, a, derived from a LLS fit to thelogarithmic-transformed data points, as described by equa-tions (2)–(5):

ln y0ð Þ � ln ytð Þ ¼ a� t (2)

Δ ln yð Þ ¼ a; for Δt ¼ 1 year (3)

R ¼ Δ yð Þ ¼ ea � 100; for Δt ¼ 1 year (4)

f ¼ y0yt

¼ e�að Þt (5)

[23] The R percent change per year (Δt= 1 year) is deter-mined from (4), and the f factor of change over the full timeperiod of the historical measurements (e.g., Δt= 50 yearsfor measurements between 1960 and 2010) is determinedfrom (5). The LLS fits are not weighted by the uncertaintiesof the individual data points due to differences in calculatingthe reported uncertainties for abundances and emissions ra-tios described above. Uncertainties associated with the rateand factor of change reflect propagation of the 1s standarddeviation of the fitted slope.[24] This analysis method is also used to determine trends

in emissions and emission ratios of precursors in bottom-upemission inventories. Annual average emissions predictedby the inventories are typically reported by mass in units ofshort tons d�1, except for CO2 from the CARB greenhousegas inventory which is reported in metric tons d�1. Molaremission ratios are calculated from annual average massemissions by converting to metric tons then dividing bymolecular weight (i.e., 0.028 kg/mol for CO, 0.046 kg/molfor NO2, and 0.044 kg/mol for CO2). Emissions and emissionratios involving unspeciated reactive organic gases (ROG) arenot converted to molar values but are instead reported on anarbitrary scale. In sections 3.1 and 3.2, we compare trends inambient measurements using airborne and ground-based fieldstudies and surface network observations to inventory emis-sion trends estimated by summing all source contributionsin the yearly CARB emission inventories for the SoCAB[California Air Resources Board (CARB), 2009, accessed

Table 5. Rate of Change (Δ) and Corresponding 1s Standard Deviation in Units of Percent Change per Year for Abundances and EmissionRatios of Precursors From the EMFAC2011 Emission Inventory Between 1990 and 2010 [CARB, 2011, Accessed January 2013] and theCARB Emission Inventory for the SoCAB Between 1975 and 2010 [CARB, 2009, Accessed January 2013]a

Measurement

EMFAC2011-LDV EMFAC2011-SG CARB Inventory CARB Inventory

(Light-Duty Gasoline Vehicles) (Light-Duty Gasoline +Heavy-Duty Diesel) (On-Road Sources) (All Sources)

Δ (% yr�1) r2 N Δ (% yr�1) r2 N Δ (%yr�1) r2 N Δ (% yr�1) r2 N

NOx �6.2� 0.3 0.99 5 �4.2� 0.7 0.91 5 �2.2� 0.5b 0.85 8 �2.1� 0.4b 0.85 8ROG �8.4� 0.2 1.00 5 �8.2� 0.2 1.00 5 �6.2� 0.4b 0.96 8 �4.4� 0.4b 0.96 8CO �8.5� 0.2 1.00 5 �8.4� 0.1 1.00 5 �5.8� 0.5b 0.96 8 �4.9� 0.4b 0.96 8CO2 1.5� 0.1 0.98 5 1.6� 0.2 0.96 5 0.4� 0.1c 0.74 20 0.9� 0.1c 0.74 20CO/CO2 �9.9� 0.3 1.00 5 �9.8� 0.3 1.00 5NOx/CO2 �7.6� 0.3 0.99 5 �5.6� 0.6 0.96 5ROG/CO2 �9.6� 0.3 1.00 5 �9.6� 0.3 1.00 5NOx/CO 2.5� 0.3 0.96 5 4.6� 0.9 0.90 5 3.7� 0.2 0.99 8 2.9� 0.2 0.99 8ROG/CO 0.1� 0.1 0.20 5 0.2� 0.1 0.43 5 �0.5� 0.1 0.62 8 0.5� 0.1 0.74 8ROG/NOx �2.4� 0.4 0.92 5 �4.2� 0.9 0.87 5 �4.1� 0.2 0.99 8 �2.3� 0.2 0.97 8

aA negative rate of change signifies a decreasing trend over time; N represents the number of data points used for linear regression.bNOx, ROG, and CO from all emissions sources in the SoCAB [CARB, 2008a, accessed January 2013].cGross CO2 emissions from the statewide greenhouse gas inventory [CARB, 2008a, accessed January 2013].


5900

January 2013]. In section 3.2, we specifically compare trendsin near-tailpipe measurements of light-duty gasoline-fueledvehicles (LDV) from roadside monitors to the EMFAC2011on-road mobile emission inventory for this subset of vehicles(Table 5) [CARB, 2011, accessed January 2013].[25] In section 4, trends in secondary oxidation products

determined from ambient measurements are compared withsimulated mixing ratios [Fujita et al., 2013] predicted usinga chemical box model.

3. Trends in Ozone and Its Precursors

3.1. Atmospheric Abundances

[26] Figure 1 illustrates the significant decrease in ozoneconcentrations observed in the SoCAB since the 1970s.The data points represent maximum 8 h average ozonemixing ratios measured basin wide and at selected individualsurface monitoring network stations in the SoCAB for a1 year period. Basin-wide measurements represent the maxi-mum value reported from any of the individual network sitesin the SoCAB in a given year, while selected individual sitesare depicted to demonstrate the progression of ozone fromcoastal to inland sites across the basin. Here we derive thefactor and rate of change in ozone from the slope of theLLS regression of the natural log of the basin-wide maximum8 h ozone values in Figure 1. The data show that ozone max-imum concentrations in the SoCAB have decreased at a rateof 2.8� 0.2%yr�1, equivalent to a decrease of a factor of 2.9

over nearly four decades. Although the maximum 8 h aver-age ozone from the individual sites is often less than thebasin-wide average, LLS fits of the long-term trends fromthe individual sites show similar rates of decrease in ozone(average of 3.3� 0.2%yr�1 for the five sites shown inFigure 1). We find that the basin-wide data are well fit by aconstant exponential decrease of 2.8% yr�1 with no indica-tion of a change in that rate between 1973 and 2010, in con-trast to recent descriptions of ozone trends [e.g., Fujita et al.,2013; Warneke et al., 2012] that have invoked a slower rateof decrease in ozone in Los Angeles after 1999. Inclusionof a constant term in the fitting expression to represent anonnegligible ozone background does not significantlyimprove the residuals to the data points in Figure 1.However, the constant rate of change must slow when futureozone concentrations approach baseline ozone levelstransported into the SoCAB.[27] The observed trend in ozone is positively correlated

with trends in its precursors. Time series plots of the annuallyaveraged abundances of NOx and CO (Figure 3) and selectVOCs (Figure 4) using data from the literature, ground-basedfield campaigns, and the Azusa and Upland SCAQMD surfacenetwork stations demonstrate large decreases in precursoremissions over the past five decades [McDonald et al., 2012;Warneke et al., 2012]. Considering all data, abundances ofNOx have decreased at an average rate of 2.6� 0.3%yr�1,resulting in a factor of 3.7 decrease over the 50 years between1960 and 2010. There are site-specific differences in theobserved rates, but we take the combined observationsfrom all locations as the most robust indication of the rateof NOx decrease in the SoCAB. The combined rate ofdecrease reported here of 2.6� 0.3% yr�1 is in agreementwith NOx emissions trends in the CARB emissions inventory[CARB, 2009, accessed January 2013], which predict anannual average decrease in NOx of 2.1� 0.4%yr�1 for allsources between 1975 and 2010 (Table 5).[28] Since 1960, CO abundances have decreased at a rate

of 5.7� 0.3% yr�1, roughly a factor of 2 more rapidly thanNOx, corresponding to a factor of 19 decrease in CO between1960 and 2010 [e.g., Warneke et al., 2012]. The overall rateof change from CO measurements in the SoCAB is consis-tent with a nationwide decrease in urban CO abundances of5.2� 0.8%yr�1 observed between 1989 and 1999 byParrish et al. [2002]. Observed CO trends in the SoCABare consistent with those predicted in the CARB emissionsinventory (e.g., annual average decrease in CO of 4.9� 0.4%yr�1 from all sources and 5.8� 0.5%yr�1 from on-roadmobile sources) [CARB, 2009, accessed January 2013].[29] For anthropogenic VOCs, an average rate of decrease

of 7.3� 0.7%yr�1, corresponding to a decrease in averageabundances by a factor of 44 over the 50 years, is determinedusing data from the Azusa and Upland SCAQMD stationsand the ground-based field measurements since 1960(Figure 4). This value, derived from two representative sites,is in quantitative agreement with a basin-wide average rate ofdecrease of 7.5%yr�1 [Warneke et al., 2012]. The annualaverage decrease in ROG of 6.2� 0.4%yr�1 between 1975and 2010 solely from SoCAB on-road mobile sources inthe CARB inventory (Table 5) is in better agreement withthe ambient observations than is the annual average decreaseof 4.4� 0.4% yr�1 determined from all sources in the CARBinventory.

0.12

4

12

4

102

4

100

CO

(pp

mv)

201020001990198019701960

10

2

46

100

2

46

1000

NO

x (p

pbv)

Figure 3. Average abundances of NOx (in units of ppbv)and CO (in units of ppmv) measured at the Azusa (solid blackcircles) and Upland (open circles, offset by 0.5 years on xaxis) surface network sites and ground-based field campaignsnear LA/Pasadena (solid blue triangles) and Claremont (openblue triangles) since 1960. Error bars represent confidencelimits based on the number of days of observations per year.LLS fits (red lines) represent long-term trends in NOx and COabundances.


5901

[30] In contrast to anthropogenic VOCs, temporal trends inisoprene (Figure 4) show no statistically significant changesince 1987 in the abundance of this biogenic VOC in theSoCAB. Additionally, no significant differences in averageabundances of isoprene were observed from the arithmeticmean of the data from summertime weekday mornings(0500 and 0900 PDT) and weekday afternoons (1200 and

1800 PDT), when emissions of biogenic VOCs are mostprominent [Guenther et al., 1993].

3.2. Emissions Ratios

[31] Although many factors have contributed to changes inambient concentrations of NOx, CO, and VOCs over theyears, decreasing abundances in the SoCAB are predomi-nantly attributed to decreasing emissions from motor vehi-cles due to increasingly strict emissions standards inCalifornia [Ban-Weiss et al., 2008; Bishop and Stedman,2008; Fujita et al., 2013; Harley et al., 2005; Lawsonet al., 1990; Lawson, 2003; McDonald et al., 2012;Warneke et al., 2012]. Large decreases in motor vehicleemissions have occurred despite a factor of 2.4 increase inpopulation (http://quickfacts.census.gov/qfd/states/06000.html) and a factor of 3 increase in fuel sales in the state ofCalifornia since the 1960s [Warneke et al., 2012]. In this sec-tion, we investigate the relative changes in precursor abun-dances and demonstrate their connection to changes inmotor vehicle emissions by examining changes in observedemissions ratios.3.2.1. Emissions Ratios to CO2

[32] We start by comparing emissions ratios to CO2

derived from near-tailpipe roadside measurements toairborne and ground-based measurements from field studies.Figure 5 illustrates changes over time in molar emissionratios of CO/CO2, NOx/CO2, and (total HC)/CO2 determinedfrom the roadside observations since 1991 and from ambientobservations from field studies since 1997 when ozone pre-cursors and CO2 were concurrently measured. As in previousstudies [Murphy et al., 2007; Parrish et al., 2002; Pollacket al., 2012], we use field observations of gas-phase NOy

when available as a more conserved tracer than NOx for de-termining emissions ratios to long-lived atmospheric speciessuch as CO and CO2. Benzene is selected for comparison ofambient measurements of speciated VOC/CO2 to roadsidemeasurements of (total HC)/CO2 due to its relatively slow re-activity with OH, thereby providing information about emis-sions independent of atmospheric processing or removal.[33] The differences between roadside and ambient mea-

surements reflect contributions from sources other than motorvehicles to total emissions of ozone precursors in the SoCAB.In Figure 5, ambient molar emission ratios of CO/CO2 andNOy/CO2 are smaller than the corresponding emission ratiosdetermined from near-tailpipe roadside measurements by anaverage percent difference of 37� 14% and 36� 9%, respec-tively. This difference indicates smaller relative contributionsof on-road motor vehicles to total CO2 emissions than to totalCO and NOx. According to 2008 California emission invento-ries [CARB, 2008a, accessed January 2013, 2008c, accessedJanuary 2013], on-road motor vehicles are responsible for53% of total CO and 49% of total NOx in the SoCAB but only35% of the total statewide CO2 emissions. Assuming that thecontribution from on-road motor vehicles to total CO2 emis-sions in the SoCAB is the same as it is statewide (35%), we ex-pect a 34% and 29% difference between roadside and ambientCO/CO2 and NOx/CO2 measurements for the SoCAB. Thesedifferences are in close accord with the observations, whichshow a difference of 37� 14% for CO/CO2 and 36� 9% forNOx/CO2, suggesting minimal variability between differentmeasurement platforms and demonstrating the relative accu-racy of the CARB emission inventories.

0.01

0.1

1

10

o-X

ylen

e (p

pbv)

0.01

0.1

1

10

Eth

ylbe

nzen

e (p

pbv)

0.1

1

10

100

Tol

uene

(pp

bv)

0.1

1

10

100

Ben

zene

(pp

bv)

0.1

2

4

1

2

4

10

Isop

rene

(pp

bv)

201020001990198019701960Year

Figure 4. Average abundances of select NMOCs (in unitsof ppbv) measured at the Azusa (solid black circles) andUpland (open circles, offset by 0.5 years on x axis) surfacenetwork sites and ground-based field campaigns near LA/Pasadena (solid blue triangles) and Claremont (open blue tri-angles) since 1960. Error bars represent confidence limitsbased on the number of days of observations per year. LLSfits (red lines) represent long-term trends in abundances ofspeciated NMOCs. NMOCs related to vehicle exhaust havedecreased at an average rate of 7.3% yr�1. No significantchange is observed for isoprene, an indicator of biogenicemissions.


5902

http://quickfacts.census.gov/qfd/states/06000.html

http://quickfacts.census.gov/qfd/states/06000.html

[34] Roadside measurements of CO/CO2 and NOx/CO2

between 1991 and 2010 and ambient measurements of CO/CO2 and NOy/CO2 between 1997 and 2010 decreased at sim-ilar rates, within the 1s uncertainties of each determination(Figure 5 and Table 4). Larger differences are observedbetween trends in roadside measurements of (total HC)/CO2 (�11.6� 1.0%yr�1) and ambient measurements ofbenzene/CO2 (�7.7� 1.3%yr�1). Due to its toxicity,benzene has been selectively and progressively removedfrom fuels over the years, accounting for its disproportionalchange relative to CO. Regulatory efforts have resulted in anationwide reduction of benzene relative to acetylene fromvehicle exhaust by 40% between 1994 and 2002, correspond-ing to a reduction in benzene emissions by roughly 56%[Fortin et al., 2005]. Fuel composition changes focused onminimizing benzene emissions between 1994 and 2002 con-tributed to the factor of 65 decrease in benzene abundances inthe SoCAB observed between 1960 and 2010 (Figure 4).This specific focus on benzene approximately accounts forthe greater decrease in ambient benzene abundances relativeto the factor of 32 decrease in toluene abundances over thesame 50 year period. Since benzene has decreased faster than

most VOCs, we expect the rate of decrease in emissions ra-tios of benzene/CO2 to represent an upper limit for decreasesin ambient VOC/CO2 ratios over time; however, (total HC)has decreased at a faster rate than benzene with respect toCO2. Discrepancies between the trends may arise from thelack of ambient benzene/CO2 data prior to implementationof regulatory efforts in 1994. Excluding the 1991 roadsidedata point from the LLS fit gives a rate of change for (totalHC)/CO2 of �8.9� 2.3% yr�1, which is in better agreementwith the decrease in ambient measurements of benzene/CO2

(�7.7� 1.3%yr�1). We also compare trends in molar emis-sion ratios to CO2 determined from roadside measurements,which targeted gasoline-fueled passenger vehicles, since1991 to those predicted by the EMFAC2011-LDV emissioninventory during summer between 1990 and 2010 (Table 5)[CARB, 2011, accessed January 2013]. Trends in roadsidemeasurements (Table 4) agree reasonably with those fromEMFAC2011-LDV (Table 5), demonstrating consistencybetween measurements and inventory emissions for light-duty vehicles. A slower decrease in NOx/CO2 ratio determinedusing EMFAC2011-SG (encompassing all on-road sources)compared to EMFAC2011-LDV shows that diesel-fueledvehicles are a significant contributor to on-road NOx emissions[Ban-Weiss et al., 2008; McDonald et al., 2012]. From thedifference in long-term trends from the EMFAC2011 inven-tory, we estimate that the presence of diesel-fueled vehicleshas slowed the decrease of NOx emissions from on-roadmobile sources by 2.1� 0.7%yr�1 consistent with the ambientmeasurements that sampled all sources in the SoCAB.3.2.2. Emissions Ratios to CO[35] Next we look at trends in emissions ratios of select

VOCs andNOx relative to CO (Figure 6). A LLS fit of ambientenhancement ratios shows a statistically significant trend of�1.2� 0.4%yr�1 since 1960 in benzene/CO but nosignificant trend (�1.8� 2.3%yr�1) in (total HC)/CO ratioin roadside measurements since 1991 (Table 4). The differ-ences are attributed to the temporal extent of the data record(e.g., ambient measurements since 1960 and roadside dataonly since 1991) as well as the faster decrease in emissionsof benzene resulting from regulatory efforts initiated in themid-1990s. The EMFAC2011-LDV emission inventory sug-gests no significant trend (+0.1� 0.1%yr�1) for ROG/CO be-tween 1990 and 2010, consistent with the lack of trend in theroadside measurements.[36] Similarly, the long-term rate of change in ambient

observations of benzene/CO can be compared with ROG/CO ratio determined for all sources in the SoCAB fromthe CARB emission inventory between 1975 and 2010(Figure 6). Inventory-based ROG/CO ratios are presentedon an arbitrary scale in Figure 6 and result in a small but sig-nificant rate of increase of 0.5� 0.1% yr�1. In contrast,ROG/CO ratio determined for on-road sources results in asmall but significant rate of decrease (�0.5� 0.1% yr�1).The long-term trends determined from the CARB inventoryare inconsistent with the larger decrease in benzene/CO(�1.2� 0.4% yr�1) from the ambient measurements,although the latter reflects the additional decrease in ben-zene due to regulatory efforts. In a similar analysis utilizinganother long-lived and well-conserved tracer, Warnekeet al. [2012] found little change in the ratio of acetylene/CO (0.3� 0.5% yr�1) since 1960 (C. Warneke, personalcommunication, 2013).

0.1

0.01

0.01

0.001

0.01

0.001

0.1

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10-5

2

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2

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1990 1995 2000 2005 2010

CO

/CO

2 CO

/CO

2

HC

/CO

2

Benzene/C

O2

NO

x/C

O2 N

Oy /C

O2

Year

Figure 5. Molar emissions ratios of CO/CO2, NOx/CO2,and HC/CO2 (left axes) determined from roadside measure-ments (green crosses) and emissions ratios of CO/CO2,NOy/CO2, and benzene/CO2 (right axes) from airborne (redsquares) and ground-based field measurements (blue trian-gles). LLS fits are indicated for roadside (dashed black line)and field (solid black line) measurements.


5903

[37] Since regulatory efforts have led to additional reductionsin benzene relative to other VOCs and CO from light-dutyvehicles, we report trends in toluene, ethylbenzene, and o-xylene to further characterize changes in gasoline-fueled motorvehicle emissions over time. Although these VOCs are shorter-lived, we use data sampled during weekday mornings between0500 and 0900 PDT to characterize VOC/CO ratios when emis-sions are at a maximum [Parrish et al., 2002] and prior to exten-sive photochemical processing, as described in section 2.2.Ambient measurements show a slight increase in VOC/COratio over time (e.g., +1.3� 0.4%yr�1 for toluene/CO,+0.8� 0.5%yr�1 for ethylbenzene/CO, and +0.5� 0.5%yr�1

for o-xylene/CO). Assuming these compounds have a com-mon source and should show similar changes, an average of+0.3� 1.0%yr�1 between 1960 and 2010 suggests there isno detectable trend in the average gasoline-fueled motor vehi-cle VOC/CO emissions ratio. This is in general agreementwith a trend of �1.7� 1.0%yr�1 determined from the rates

of change in abundances of VOCs (�7.3� 0.7%yr�1) andCO (�5.7� 0.3%yr�1) using the combined observations(Table 4). Warneke et al. [2012] found the ratios to COrelatively constant for all VOCs in the LA basin since 1960.The long-term rate of increase of 0.5� 0.1% yr�1 forROG/CO in the SoCAB determined from the CARB emis-sion inventory between 1975 and 2010 is in agreement withthe combined observations of VOC/CO (+0.3� 1.0% yr�1)from the ambient measurements since 1960.[38] Emissions ratios of NOy/CO from roadside and ambi-

ent measurements are also shown in Figure 6. The observedincrease in NOx/CO emissions ratio follows from the fasterdecrease in CO emissions (average of 5.7%yr�1) comparedto the decrease in NOx (average of 2.6%yr�1) even thoughthe atmospheric abundances of both have decreased signifi-cantly since 1960. LLS fit of the combined ambient and road-side measurements shows a positive trend in the NOx/COemissions ratio of 4.9� 0.4%yr�1 between 1960 and 2010

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1960 1970 1980 1990 2000 2010

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Ox

Ethylbenzene/N

Ox

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(tot

al H

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x, R

OG

/NO

x

Year

Figure 7. Ambient VOC/NOx ratio (units of ppbv/ppbv,right axes scales) for airborne (red squares), ground-basedfield measurements near Azusa, Pasadena, and LA (solidblue triangles) and Claremont (open blue triangles), and sur-face network measurements for Azusa (solid black circles)and Upland (open black circles). Roadside measurements of(total HC)/NOx (green crosses on left axis); emission ratiosof ROG/NOx from the CARB emission inventory for allsources (purple symbols) and on-road sources (orange sym-bols) are presented on an arbitrary scale. Solid lines representthe LLS fits to the combined ambient measurements; dashedlines represent LLS fits to the inventory data.

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1960 19801970 1990 2000 2010Year

Figure 6. Ambient emissions ratios of VOC/CO and NOx/CO (in units of ppbv/ppbv) determined from airborne mea-surements (red squares), ground-based field measurementsnear Azusa, Pasadena, and LA (solid blue triangles) andClaremont (open blue triangles), and surface network mea-surements for Azusa (solid black circles) and Upland (openblack circles). Emission ratios of (total HC)/CO and NOx/CO (green crosses) from roadside measurements and ROG/CO and NOx/CO from the CARB emission inventory for allsources (purple symbols) and on-road sources (orange sym-bols) are also shown. Emission ratios of NOx/CO from thevarious measurements are presented on a common scale.Ambient measurements of VOC/CO correspond with theright axis scale, roadside measurements of (total HC)/COcorrespond with the left axis scale, and ROG/CO from the in-ventory is presented on an arbitrary scale. Solid lines repre-sent the LLS fit to the combined ambient measurements;dashed lines represent LLS fits to the inventory data.


5904

(Table 4). This value is larger than the trend of 3.3� 0.5%yr�1 based on the relative rates of decrease in NOx and COabundances. The rate of increase in NOx/CO from the ambi-ent field measurements (5.2� 1.4% yr�1) is in agreementwith that determined from the roadside measurements butlarger than the long-term rate of change in NOx/CO ratio pre-dicted between 1975 and 2010 for the SoCAB by the CARBemission inventory (3.7� 0.2%yr�1 for on-road sources and2.9� 0.2%yr�1 for all sources) (Table 5).[39] Absolute emission ratios of NOx/CO predicted by the

CARB emission inventory, which are included in Figure 6,are larger than that determined from the combined observa-tions. Overestimates of NOx/CO ratio from the inventorydecrease from a factor of 4.3 for all sources and 2.8 for on-road sources in 1990 to a factor of 1.9 and 1.6, respectively,in 2010. These differences must reflect significant error(s) inthe inventory. In contrast, top-down estimates of emissionsratios of NOy/CO for weekdays in 2002 and 2010 in the LAbasin using an inverse model [Brioude et al., 2012] show bet-ter agreement with absolute emission ratios determined fromthe field measurements (e.g., NOy/CO ratios simulated for2002 and 2010 are underestimated by an average factor of1.5). The long-term rate of change in measured weekdayambient NOx/CO is also in agreement with that determinedfrom the EMFAC2011-SG inventory (4.6� 0.9%yr�1),which combines emissions from gasoline-fueled and diesel-fueled vehicles [Pollack et al., 2012]. However, there is lessagreement between the long-term increase in roadside mea-surements of NOx/CO from light-duty vehicles (5.9� 1.3%yr�1) and the rate of increase determined from theEMFAC2011-LDV inventory (2.5� 0.3%yr�1).

[40] Here we have utilized long-lived, well-conservedspecies to make direct correlations between abundancesand emissions ratios measured in the SoCAB. Consistencybetween trends derived from near-tailpipe roadside andfrom ambient measurements confirms on-road motorvehicles as the predominant source of CO, VOC, and NOx

emissions in the SoCAB. Agreement between trends inmeasured emissions ratios and atmospheric abundancesdemonstrates that changes in ambient concentrations inthe SoCAB continue to reflect changes in emission ratesof precursor species. This analysis has shown that allof the long-term trends in emissions, except for NOx,are underestimated by the CARB inventory for all sourcesin the SoCAB. In contrast, all of the long-term rates ofchange predicted by EMFAC2011-LDV, with the exceptionof NOx/CO ratio, agree reasonably with the long-termtrends determined from the roadside measurements oflight-duty vehicles emissions. Differences between long-term trends from the measurements and emission invento-ries suggest that statistically significant errors still exist inthe inventories.3.2.3. VOC/NOx Ratio[41] Larger reductions in emissions of VOCs relative to

NOx over time have resulted in decreasing VOC/NOx

emission ratios in the SoCAB over the past five decades.Data from the monitoring network and from intensivefield measurements (Figure 7) show the trends in toluene,ethylbenzene, and o-xylene emissions relative to NOx.These three VOCs are selected as compounds characteris-tic of vehicle exhaust with OH reaction rate coefficientssimilar to that for NO2 [Sander et al., 2006], such that

2010 2005 2000 1995

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8-h

r av

erag

e O

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erag

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6 80.01

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Toluene/NOx ratio0.001

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2 4 6 80.01

2 4

o-Xylene/NOx ratio

Figure 8. Maximum 8 h average ozone (in units of ppbv) versus VOC/NOx ratios (in units of ppbv/ppbv)from the Azusa (solid symbols) and Upland (open symbols) network monitoring sites. Decreasing ozone iscorrelated with decreasing VOC/NOx ratio from 1994 (red) to 2010 (purple); data from 1975 (black) arealso shown where available. Solid black lines represent LLS fits of the data points; also reported are theR2 values determined from each fit.


5905

VOC/NOx emissions ratios should be approximately pre-served during processing by OH. Additionally, as notedabove, we minimize chemical processing of the emissionsprior to analysis by selectively using data from weekdaymornings. A trend of �4.8� 0.9% yr�1 for these VOC/NOx ratios is derived from network and field measure-ments, consistent with the value of �4.5� 0.8% yr�1

calculated from observed trends in VOC and NOx abun-dances. This trend is significantly larger than�2.3� 0.2% yr�1 in the unspeciated ROG/NOx emissionsratio predicted by the CARB inventory for all sources inthe SoCAB between 1975 and 2010. The discrepancybetween observed and inventory values is consistent withFujita et al. [2013], who reported that ambient measuredNMOC/NOx ratios from SCAQS 1987, SCOS 1997, andfield measurements in 2009 are about a factor of 2 higherthan the emission inventory ratios. Although a trend of�2.4� 0.4% yr�1 in ROG/NOx ratio is predicted by theEMFAC2011-LDV emission inventory [CARB, 2011,accessed January 2013] between 1990 and 2010, roadsidedata since 1991 show no significant trend (2.0� 2.7%

yr�1) in (total HC)/NOx emissions ratio (Table 4).Trends in ambient VOC/NOx ratios (average of �4.80.9% yr�1) are in agreement with changes of �4.20.9% yr�1 in ROG/NOx ratio determined from theEMFAC2011-SG emission inventory, which reflects com-bined emissions from light-duty gasoline-fueled andheavy-duty diesel-fueled vehicles.

4. Causes of Decreasing Ozone in the SoCAB

[42] Ozone formation in the SoCAB strongly depends onabundances of anthropogenic VOCs and NOx and on theVOC/NOx ratio [Fujita et al., 2003; Fujita et al., 2013;Lawson, 2003; Pollack et al., 2012]. Figure 8 shows positivecorrelations between decreasing maximum 8 h ozone con-centrations and decreasing VOC/NOx ratio since 1975 fromthe Azusa and Upland surface network data sets. However,the causal relationships between decreasing ozone and de-creasing precursor emissions, and their ratios, remainunclear. In the following sections, we use historical measure-ments of secondary oxidation products to better identify themajor causes of decreasing ozone in the SoCAB. We analyzeambient measurements of secondary pollutants from sum-mertime weekday afternoons between 1200 and 1800 PDTwhen photochemical processing is well advanced and theplanetary boundary layer is well developed. We assume neg-ligible influence from any changes in meteorological condi-tions that might affect average atmospheric residence timesin the SoCAB, dilution, pollutant carryover, transport, anddeposition over those 50 years. As discussed in sections 4.2and 4.3, this approach is supported by airborne measure-ments from CalNex 2010, ARCTAS 2008, and ITCT 2002,which show no statistically significant difference in NOy/CO and NOy/CO2 emissions ratios throughout the samplingduration of each flight (typically 1000 to 1800 PDT) and be-tween the eastern and western geographic portions of theSoCAB over the past decade.

4.1. Long-Term Trends in NOx Oxidation Products

[43] Atmospheric PAN is a result of HOx radical chemistryand is positively correlated with ozone production [Pollacket al., 2012; Roberts et al., 2007; Roberts et al., 1995;Sillman, 1995]. Thermal decomposition of PAN regeneratesRO2 and NO2 and promotes continued ozone production. Incontrast, formation of HNO3 removes OH radicals from thechain reactions that produce ozone, and thus atmosphericHNO3 is a result of chemical reactions that terminate theozone formation cycle. The abundance of PAN relative toHNO3 provides an indication of the balance between propa-gation and termination of the catalytic ozone formation cycle.[44] The temporal trends of the average abundances of

PAN and HNO3 since 1973 from the available airborne andground-based measurements are illustrated in Figure 9.PAN maxima prior to 1973 reported in the literature are alsoshown in the figure, although they are not included in the de-termination of the long-term trend. Although concentrationsof secondary pollutants may vary significantly with locationin the SoCAB, the data in Figure 9 show no systematic differ-ence in abundances of PAN measured at these selected loca-tions in the SoCAB. Measured abundances of PAN exhibitan average trend of �9.3� 1.1%yr�1 since 1973 (Table 6),corresponding to a factor of 34 decrease between 1973 and

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1960 1970 1980 1990 2000 2010Year

PA

N/H

NO

3H

NO

3 (p

pbv)

PA

N (

ppbv

)

Figure 9. Changes in average abundances of PAN andHNO3 and enhancement ratios of PAN/HNO3 since 1973using data from airborne (red squares) and ground-based fieldstudies near Los Angeles (solid blue triangles) and Claremont(open blue triangles). Observed PAN maxima between 1960and 1970 are included in the upper plot but excluded fromthe LLS fit (solid black lines); however, the extrapolatedLLS fit to 1960 (dashed black lines) is in agreement with thoseobservations. Values predicted by box model simulations[Fujita et al., 2013] (green symbols and dashed lines) areshown in comparison to the measurements.


5906

2010. PAN was first identified as a particularly important eyeirritant in Los Angeles smog [Leighton, 1961]. PAN abun-dances have decreased much more rapidly than ozone (e.g.,factor of 34 reduction in PAN versus factor of 2.9 reductionin O3 over the same 37 year period), demonstrating muchgreater progress in reducing some compounds of concernfor air quality compared to O3.[45] Previous studies [Kelly, 1992; Russell et al., 1988b;

Spicer, 1983] have shown direct correlation between reduc-tions in PAN, which is produced from VOC oxidation inthe presence of NOx, and reductions in both VOC and NOx

precursors. The observed trend in PAN since 1973 of�9.3� 1.1% yr�1 is quantitatively similar to the rate ofchange in the product of the trends in its precursors(�9.7� 0.7% yr�1), calculated from the sum of the slopesof the long-term trends of VOC and NOx since 1960(Table 4) and using equation (4). The difference may beattributed to our selection of VOCs for the combined trendreported in Table 4, which does not include acetaldehyde,the direct precursor to PAN formation. Analysis of the trendin acetaldehyde results in a decrease of 7.3� 1.6%yr�1 inthe SoCAB, although this is based solely on measurementsafter 1980. A similar decrease is observed for formaldehyde,which serves as an indicator for formation of secondaryorganic products. Analysis of formaldehyde maxima[Grosjean, 1982] dating back to 1960 shows an averagetrend of �7.6� 1.0% yr�1, consistent with that calculatedby averaging trends in directly emitted benzene, toluene,ethylbenzene, and o-xylene (�7.3� 0.7% yr�1) (Table 4).The observed PAN trend since 1973 (�9.3� 1.1% yr�1) isin agreement with the product of the decrease in abundancesof formaldehyde and NOx (10.0� 1.0% yr�1).[46] Average abundances of HNO3 have changed at a rate

of �3.0� 0.8%yr�1 (Table 6), corresponding to a factor of3.1 decrease between 1973 and 2010. Reductions in HNO3

concentrations have been found to be directly proportionalto decreases in NOx [Kelly, 1992; Russell et al., 1988b;Spicer, 1983]. The trend in HNO3 agrees within statisticaluncertainty with the average rate of decrease determined forNOx (�2.6� 0.3%yr�1). As found for PAN, average

HNO3 abundances in Figure 9 show no dependence on thelocations selected to represent the SoCAB in this analysis.The faster rate of decrease in PAN relative to HNO3 since1973 results in a trend in enhancement ratios of PAN/HNO3 of �7.3� 1.6% yr�1. Owing to the positive correla-tions of PAN with ozone production and HNO3 with radicaltermination, long-term decreases in the relative concentra-tions of PAN and HNO3 indicate that decreasing ozone con-centrations in the SoCAB are a direct result of decreasingphotochemical production of ozone due to decreasing abun-dances of VOC and NOx precursors.[47] Also shown in Figure 9 are abundances of PAN and

HNO3 simulated for 1985, 1995, and 2005 in the SoCABusing a chemical box model [Fujita et al., 2013]. While sim-ulated abundances of PAN are consistent with the measuredvalues, simulated abundances of HNO3 are overestimatedby roughly a factor of 3. The model also predicts significantlylarger rates of decrease in PAN and HNO3 than the apparenttrends in the ambient measurements (Table 6).

4.2. Change in Ozone Production Efficiency

[48] In this section, we investigate the contribution ofchanges in ozone production efficiency (OPE) to the ob-served decreases in ozone abundances over time. We takethe slope of a LLS fit of O3 versus the sum of the majorNOx oxidation products (PAN+HNO3) to provide a measureof ozone produced per NOx oxidized [Trainer et al., 1993].Figure 10 shows a scatter plot of O3 versus (PAN+HNO3)using the available field measurements in the SoCAB since1973. LLS fits result in large variability in the slopes deter-mined from each data set. Given the variability in fittedslopes between data sets, this analysis finds no significanttrend in derived OPE between 1973 and 2010. LLS slopesfrom scatter plots of odd oxygen (Ox=O3 +NO2) versus

Table 6. Rate of Change (Δ) and Corresponding 1s StandardDeviation in Units of Percent Change Per Year for Abundancesand Enhancement Ratios of Secondary Oxidation ProductsDetermined From the Airborne and Ground-Based FieldMeasurements Since 1973 and Values Predicted by Fujita et al.[2013] for 1985, 1995, and 2005 Using a Chemical Box Modela

Measurement

Field Observations Model Predicted

Δ (%yr�1) r2 N Δ (% yr�1) r2 N

PAN �9.3� 1.1 0.82 15 �13.5� 1.1 0.99 3HNO3 �3.0� 0.8 0.47 16 �5.3� 1.0 0.96 3PAN/HNO3 �7.3� 1.6 0.74 11 �8.6� 0.1 1.00 3O3/(PAN+HNO3) 0.8� 1.4 0.04 10 �1.1� 0.6b 0.75 3Ox/(PAN+HNO3) �1.4� 1.3 0.14 9NOx/NOy �0.9� 0.9 0.77 10(PAN+HNO3)/NOy 2.2� 0.5 0.68 10 �3.0� 2.2 1.00 3PAN/NOy �2.0� 1.4 0.20 10 �10.6� 0.5 1.00 3HNO3/NOy 3.3� 1.0 0.56 10 �2.3� 0.4 0.97 3

aA negative rate of change signifies a decreasing trend over time; N repre-sents the number of data points, corresponding to years of data, used forlinear regression.

bLLS fit of values predicted for O3/(NOy-NOx) [Fujita et al., 2013].

00

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20 40 60 80 100 0 5 10 15 20 25

2010 2000 1990 1980 1970

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(ppb

v)

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Figure 10. Correlation plots of O3 versus (PAN+HNO3)using airborne and ground-based measurements from fieldstudies between 1973 (red) and 2010 (purple), with the rightplot showing an expanded view of the region inside thedashed boxed from the left plot. Ground-based measure-ments include data reported by Spicer [1977b], Tuazonet al. [1981], Hanst et al. [1982], and Grosjean [1986] andfrom the SCAQS (1987), LAAFRS (1993), SCOS (1997),and CalNex (2010) field campaigns; airborne measurementsinclude data from ITCT (2002), ARCTAS-CARB (2008),and CalNex (2010). Solid lines represent LLS ODR of eachdata set corresponding by color; a time series of the slopesis presented in Figure 11.


5907

(PAN+HNO3) are also shown in Figure 11. Ox has beeninterpreted in previous studies as a more conserved measureof net oxidant production [Johnson, 1984; Murphy et al.,2007; Pollack et al., 2012; Tonse et al., 2008] and is usedhere to isolate the relative contributions from titration andnet photochemical production to observed ozone levels. Nodetectable trend in O3/(PAN+HNO3) and Ox/(PAN+HNO3)suggests that over the 37 years spanned by the data, OPEhas remained constant within our ability to quantify from theavailable data. Model-predicted ratios for O3/(NOy-NOx)reported by Fujita et al. [2013], also shown in Figure 11,agree well with the measured ratios and further indicate nodetectable change in OPE over time.[49] The above calculations assume no differential loss of

NOy species. However, OPE interpreted from these slopeswill be overestimated if a significant fraction of any gas-phase NOy species, e.g., HNO3, is removed from the atmo-sphere prior to measurement [Ryerson et al., 1998; Traineret al., 1993]. Although minimal loss of NOy species in theSoCAB is justified by the minimal change observed in ratiosof NOy/CO and NOy/CO2 over the spatial extent of the basinand the temporal extent of sampling using the airborne datasets, we also consider the influence of loss of HNO3 on thederived OPE. Since HNO3 is highly soluble, it is subject toremoval by wet and dry deposition, and it can also form am-monium nitrate aerosol. We have recalculated OPE assuming50% loss of HNO3 prior to measurement, which results inlarger positive biases in OPE for years between 1990 and

2010 where HNO3 is a larger fraction of NOy. Even underthis extreme example, HNO3 loss does not significantly af-fect the long-term LLS fit. With or without HNO3 loss, wedetect no significant change in OPE between 1973 and2010 based on analysis of the ambient data.[50] The lack of a discernible trend in derived OPE

since 1973 (Figure 11 and Table 6) contradicts an expecteddecrease due to the decreasing VOC/NOx ratio. Studies ofweekday-weekend differences in ozone in the SoCAB haveshown decreasing OPE with decreasing VOC/NOx ratio onweekdays versus weekends [Fujita et al., 2013; Pollacket al., 2012]. However, the observed changes in VOC/NOx

ratio in these studies are primarily driven by weekendreductions in NOx with little weekday-weekend differencein VOCs. Past studies [Lin et al., 1988; Liu et al., 1987;Roberts et al., 2007; Roberts et al., 1995; Ryerson et al.,1998; Ryerson et al., 2001; Trainer et al., 1993] haveshown that the amount of ozone produced per NOx oxidizeddepends nonlinearly on NOx and VOC concentrations,such that OPE at a fixed VOC/NOx ratio increases withdecreasing abundances of NOx. Here we suggest that thenet effect of decreasing NOx abundances and simulta-neously decreasing VOC/NOx ratio over time in theSoCAB have effectively canceled, resulting in minimalchange in OPE over the past 50 years.

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(PA

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/NO

yN

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NO

y

Figure 12. Changes in the fraction of unreacted NOx andNOx oxidation products (PAN+HNO3) relative to total reac-tive nitrogen (NOy) using data from airborne (red squares)and ground-based (blue triangles) field studies near LosAngeles (solid blue triangles) and Claremont (open blue tri-angles). Values predicted by box model simulations [Fujitaet al., 2013] (green symbols and dashed lines) are shown incomparison to the measurements. Assuming no loss of NOy

species, LLS fits (solid black lines) of the long-term trendsshow an average decrease in unreacted NOx (�0.9% yr�1)and an average increase in NOx oxidation products(2.2% yr�1) since 1973.

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Figure 11. Changes in O3/(PAN+HNO3) and Ox/(PAN+HNO3) since 1973 using data from airborne (redsquares) and ground-based (blue triangles) field studies nearLos Angeles (solid blue triangles) and Claremont (open bluetriangles). Values predicted by box model simulations[Fujita et al., 2013] (green symbols and dashed lines) areshown in comparison to the measurements. Assuming noloss of NOy species, LLS fits show no significant changesince 1973 (solid black lines).


5908

4.3. Changes in Rate of Photochemical Processing

[51] Assuming atmospheric mixing and residence times inthe SoCAB have not changed in the last 50 years, changes inthe fraction of oxidized NOx relative to NOy indicate differ-ences in the rate of photochemical processing of NOx overtime [Pollack et al., 2012]. Figure 12 presents the data since1973 in two coupled ways: the measured NOx to NOy ratio inthe top panel illustrates changes in the average fraction ofunreacted NOx, while measured (PAN+HNO3) to NOy ratioin the bottom panel illustrates changes in the average fractionof NOx oxidation products to NOy. As discussed in section2.1.3, measured gas-phase NOy data are used where avail-able; the sum of individually measured gas-phase NOy con-stituent species (

PNOy; typically NOx +PAN+HNO3) is

used otherwise. Alkyl nitrates were not measured in mostof the field studies used in this analysis and thus are not incor-porated into

PNOy but are assumed to be a relatively small

contribution to total NOy in the Los Angeles basin. LLS fitto these data suggests a statistically insignificant trend of�0.9� 0.9%yr�1 in the fraction of unreacted NOx in theSoCAB between 1973 and 2010 (Table 6). On the otherhand, we observe a significant increase in the fraction ofoxidized NOx of 2.2� 0.5%yr�1. This result is robust torecalculation of the (PAN+HNO3)/NOy trend assuming50% loss of HNO3 prior to gas-phase measurement andsuggests that this trend is insensitive to the fraction ofHNO3 in NOy. In contrast, ratios of (PAN+HNO3)/NOy

derived from box model simulations in Fujita et al. [2013]

show a decreasing trend in the fraction of oxidized NOx

(Figure 12 and Table 6) and are inconsistent with the increas-ing trend derived from the ambient data.[52] The ambient data suggest that atmospheric photo-

chemical processing of NOx in the SoCAB has increasedover the 37 years spanned by the available measurements.The increase in the fraction of oxidized NOx since 1973 indi-cates more rapid processing of NOx as a result of the largechanges in amount and proportion of emissions in the LosAngeles basin. Along with faster photochemical processing,emissions changes have resulted in increasingly favoredproduction of HNO3 over time, demonstrated by a derivedpositive trend of 3.3� 1.0%yr�1 in HNO3/NOy and a trendof �2.0� 1.4%yr�1 in PAN/NOy (Figure 13 and Table 6).[53] Differences between measured and simulated ratios

[Fujita et al., 2013] of HNO3/NOy and PAN/NOy are alsoillustrated in Figure 13. The differences in PAN/NOy ratiolikely reflect the model’s overestimate of HNO3.

5. Summary

[54] We quantify long-term trends in abundances andemissions ratios of precursors and secondary oxidationproducts in the SoCAB using data from a surface monitoringnetwork, roadside monitors, ground-based field sites, andresearch aircraft. Consistency in average abundances andemission ratios determined from the various platforms overthe past 50 years permits quantification of long-term trendsin emissions in the South Coast Air Basin. Agreementbetween near-tailpipe measurements by roadside monitors,basin-wide measurements from aircraft, and local assessmentsfrom ground-based measurements and surface network sitesconfirms motor vehicles as the predominant source of precur-sor emissions in the SoCAB. Differences between long-termtrends determined from the ambient measurements and emis-sion inventories suggest that significant errors still exist inthe inventories. A decrease in VOC/NOx ratio is observedsince the 1960s and is a direct result of the faster decrease inVOC emissions compared to NOx. Decreasing maximum 8hozone concentration in the SoCAB is positively correlatedwith decreasing ozone precursor abundances and VOC/NOx

ratio over the past 50 years. The well-established connectionbetween O3 and PAN production allows use of the trends inNOx oxidation products as a tool for relating long-termchanges in ozone production to changes in precursor emis-sions. The observed decreases in NOx oxidation products,PAN and HNO3, over time are consistent with decreasingVOC and NOx abundances. No change in enhancement ratiosof O3 and (O3 +NO2) to (PAN+HNO3) over time suggests nodetectable trend in ozone production efficiency since 1973,although there is wide variability in the results from differentdata sets. The trend in NOx oxidation products demonstratesan increase in the fraction of oxidized NOx since 1973,suggesting that atmospheric oxidation rates of NOx haveincreased over time as a result of the emissions changes inthe SoCAB. Trends in the fractions of PAN and HNO3 relativeto NOy show that HNO3 production has been increasinglyfavored over time.[55] This analysis demonstrates that significant reductions

in secondary pollutants have been the direct result of reducedprecursor emissions, with the largest changes related to in-creased emission standards and improved technology in

0.1

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1960 1970 1980 1990 2000 2010Year

PA

N/N

Oy

HN

O3/

NO

y

Figure 13. Changes in the fraction of PAN and HNO3 rela-tive to total reactive nitrogen (NOy) using data from airborne(red squares) and ground-based (blue triangles) field studiesnear Los Angeles (solid blue triangles) and Claremont (openblue triangles). Values predicted by box model simulations[Fujita et al., 2013] (green symbols and dashed lines) areshown in comparison to the measurements. Assuming no lossof NOy species, LLS fits (solid black lines) of the long-termtrends show an average decrease in PAN (�2.0%yr�1) andan average increase in HNO3 (3.3%yr�1) since 1973.


5909

motor vehicles. Through the decades, VOCs were the initialprimary target for emissions reductions in the SoCAB,followed more recently by increasing efforts to controlNOx. As shown in our assessment and previous analyses,these control measures have been effective in reducing ambi-ent ozone levels over the past 50 years. Stricter emissionsstandards for diesel-fueled vehicles are designed to furtherreduce NOx emissions [CARB, 2008b; McDonald et al.,2012]. Overall, the observations from this analysis suggestthat reductions in ozone and other secondary pollutants inthe SoCAB over the past 50 years are a direct result ofdecreasing abundances of VOCs and NOx and VOC/NOx

ratio and that ozone continues to be responsive to local emis-sions control strategies in the Los Angeles basin.

[56] Acknowledgments. The authors thank L. Dolislager, B. Weller,and D. Oda from the California Air Resources Board for providing historicaldata from the SCAQMD and PAMS surface monitoring networks and vari-ous field studies in the SoCAB. We also thank G. Bishop and D. Stedmanfor help with conversion and interpretation of the roadside measurements.

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